1. Field of the Invention
This invention relates to a method of producing an amorphous magnetic film, and more particularly to a method in which a laser beam is irradiated to a magnetic metal target so as to generate an evaporant which is deposited on a substrate so as to form a magnetic film having a composition identical to that of the target.
2. Description of Related Art
Heretofore magnetism has been widely applied to fields such as data recording and remote controlling. Specifically, magnetic thin films have been very important for improving functions of magnetic recording devices and magnetic sensors, for example. Various attempts and studies have been made in order to improve such magnetic thin films.
Among a variety of magnetic thin films, amorphous magnetic films have been known which demonstrate such excellent properties as a high magnetic permeability, a small coercive force, and a small magnetic anisotropy. Therefore use of the amorphous magnetic films enables acceleration of the data recording and precise detection of magnetism. These films have been considered optimum as a component not only for magnetic recording materials but also for torque sensors and pressure sensors which make use of the magnetism.
Both of the coercive force and magnetic anisotropy strongly depend upon a degree of crystallinity of materials. The higher crystallinity of the materials, the larger the coercive force and magnetic anisotropy. When the amorphous magnetic film has high degree of amorphousness, both of the coercive force and magnetic anisotropy can be decreased. In addition, since the magnetic permeability also depends upon the composition of the magnetic film materials, it is necessary to control the composition of the films accurately.
Such amorphous magnetic films have been produced by methods such as vacuum evaporation and sputtering according to kinds of thin films to be produced.
With the vacuum evaporation method, a thin film is deposited on a substrate by evaporation from a target consisting of a magnetic material, and is heated in a vacuum by a heater, for example. With the sputtering method, plasma of inert gas such as argon is formed in a chamber so as to irradiate plasma particles (ions) to a target, eject atoms or molecules from the surface of the target, and deposit the atoms or molecules on a substrate.
With these methods, an amorphous magnetic film is produced on the surface of the substrate as desired when the target has a predetermined composition and a speed for cooling the thin film on the substrate is determined at a value not to cause crystallization.
Generally the amorphous magnetic films have magnetic permeability, coercive force, magnetic anisotropy, magnetic distortion and so forth which vary with components and composition of the materials. The component and composition of the amorphous magnetic films are determined according to the field of application.
Therefore it is essential to produce an amorphous magnetic film whose component and composition are suitable to demonstrate desired magnetic properties.
When producing amorphous magnetic films by a conventional vacuum evaporation method, it is required to heat and evaporate a plurality of metallic atoms.
In such a case, it is conceivable to use an alloy target and to evaporate metallic atoms in the alloy. Since the metallic atoms composing the alloy have however different vapor pressures, it is very difficult to produce an alloy film having a desired composition. Specifically, when metals composing the alloy are separately heated by multiple-independent sources, composition of an evaporated film can be controlled as desired, but a device therefore becomes large inevitably.
In the vacuum evaporation method, a thin metallic film is deposited by evaporation from a heated target material. Evaporated particles reaching the substrate have a small kinetic energy, taking a considerable time to be cooled to the temperature of the substrate. Therefore, only limited magnetic metals or magnetic alloys can be made amorphous. In addition, the kinetic energy of the particles reaching the substrate is too small to stick the deposited film to the substrate reliably.
Many of existing amorphous magnetic films are usually made of alloys which can become amorphous at a relatively low cooling speed.
With the sputtering method, ions, which are generated by plasma or discharge of gas such as argon, are irradiated onto a target so as to evaporate the target material. This method is not applicable in a high vacuum, being disadvantageous in that impurities in a deposited film cannot be removed sufficiently. Specifically, impurities in the inert gas or the inert gas itself often mix in the deposited film. In such a case, the amorphous magnetic film cannot have the desired composition and cannot maintain the desired magnetic properties.
When producing an amorphous magnetic alloy film, it is required to use an alloy target whose composition is identical to that of a film to be produced. However the amount of atoms to be sputtered by inert gas varies with the atoms. In addition, ratios of the sputtered particles reaching the substrate depend upon the kinds of atoms. Therefore the composition of a deposited thin film sometimes differs from the composition of the target, thereby making it impossible to obtain a film having the desired properties.
With the sputtering method, a kinetic energy of particles to be deposited on the substrate is large compared with that of the vacuum evaporation method, so that the film can stick to the substrate closely. However since a temperature of the substrate is raised by plasma or discharge of the inert gas, a sufficient cooling speed cannot be attained. Only limited magnetic metals or alloys can be made amorphous.
In the foregoing methods, it is required to know the difference of compositions between the target and a film to be produced so as to obtain an amorphous magnetic film of the desired composition.